Receivers introduce different kind of non-idealities, which disturb the reception. These non-idealities are typically corrected with several different methods, which are digital or analogue or their mixture. Some of the methods use test signals to calibrate, some do not.
Receiving a multi-carrier signal, or a signal with high-order modulation, requires a high image rejection from an analog receiver. In the analog receiver, which is, for example, based on direct conversion or digital-IF (intermediate frequency) architectures, the image rejection is limited by the I/Q (inphase/quadrature) imbalance. The most important factors deteriorating the I/Q balance are a LO (local oscillator) phase error, i.e., deviation of the LO signals provided to the I- and Q-branch mixers from a perfect phase quadrature, a gain difference between components in the analog I- and Q-branches, and a difference in corner frequencies of low-pass filters in the I- and Q-branches resulting in frequency-dependent gain and phase imbalance.
Previously (per the prior art), the I/Q balance calibration typically needed specific test signals in order to perform the calibration. Also, analog baseband filter corner frequency calibration has been done by separate calibration methods, for example, based on an RC time constant measurement.
Another prior art approach to deal with the I/Q imbalance is to estimate the imbalance and then compensate it by digital processing. Various such compensation algorithms have been proposed. Of particular interest here is the algorithm described in the US Patent Application US 2004/00822300 “Digital Imbalance Correction Method and Device” by Hans-Otto Scheck. In that invention, an FFT (fast Fourier transform) is proposed to be used for estimating the gain imbalance by using a specific test signal (not a noise) as a stimuli.
The US Patent application US 2004/00822300 “Method and Apparatus Providing Calibration Technique for RF Performance Tuning” by Pauli Seppinen et al. describe receiver calibration methods, and in particular, in one of the embodiments the center frequency is calibrated by utilizing a noise to locate the right calibration word for LNA (low noise amplifier) center frequency tuning.